Programmable microelectronic devices and methods of forming and programming same

ABSTRACT

A microelectronic programmable structure and methods of forming and programming the structure are disclosed. The programmable structure generally include an ion conductor and a plurality of electrodes. Electrical properties of the structure may be altered by applying a bias across the electrodes, and thus information may be stored using the structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/267,079,entitled PROGRAMMABLE MICROELECTRONIC DEVICES AND METHODS OF FORMING ANDPROGRAMMING THE SAME, filed Oct. 8, 2002, which is a divisional ofapplication Ser. No. 09/502,915, entitled PROGRAMMABLE MICROELECTRONICDEVICES AND METHODS OF FORMING AND PROGRAMMING THE SAME, filed Feb. 11,2000, which claims the benefit of U.S. Provisional Application Ser. No.60/119,757, filed Feb. 11, 1999 and International Application SerialNumber PCT/US98/25830, filed Dec. 4, 1998.

FIELD OF INVENTION

The present invention generally relates to microelectronic devices. Moreparticularly, the invention relates to programmable microelectronicstructures suitable for use in integrated circuits.

BACKGROUND OF THE INVENTION

Memory devices are often used in electronic systems and computers tostore information in the form of binary data. These memory devices maybe characterized into various types, each type having associated with itvarious advantages and disadvantages.

For example, random access memory (“RAM”) which may be found in personalcomputers is volatile semiconductor memory; in other words, the storeddata is lost if the power source is disconnected or removed. Dynamic RAM(“DRAM”) is particularly volatile in that it must be “refreshed” (i.e.,recharged) every few microseconds in order to maintain the stored data.Static RAM (“SRAM”) will hold the data after one writing so long as thepower source is maintained; once the power source is disconnected,however, the data is lost. Thus, in these volatile memoryconfigurations, information is only retained so long as the power to thesystem is not turned off. In general, these RAM devices may be expensiveto manufacture and consume relatively large amounts of energy duringoperation of the devices. Accordingly, improved memory devices suitablefor use in personal computers and the like are desirable.

CD-ROM and DVD-ROM are examples of non-volatile memory. DVD-ROM is largeenough to contain lengthy audio and video information segments; however,information can only be read from and not written to this memory. Thus,once a DVD-ROM is programmed during manufacture, it cannot bereprogrammed with new information.

Other storage devices such as magnetic storage devices (e.g., floppydisks, hard disks and magnetic tape) as well as other systems, such asoptical disks, are non-volatile, have extremely high capacity, and canbe rewritten many times. Unfortunately, these memory devices arephysically large, are shock/vibration-sensitive, require expensivemechanical drives, and may consume relatively large amounts of power.These negative aspects make these memory devices non-ideal for low powerportable applications such as lap-top and palm-top computers, personaldigital assistants (“PDAs”), and the like.

Due, at least in part, to a rapidly growing numbers of compact,low-power portable computer systems in which stored information changesregularly, read/write semiconductor memories have become increasinglydesirable and widespread. Furthermore, because these portable systemsoften require data storage when the power is turned off, non-volatilestorage device are desired for use in such systems.

One type of programmable semiconductor non-volatile memory devicesuitable for use in such systems is a programmable read-only memory(“PROM”) device. One type of PROM, a write-once read-many (“WORM”)device, uses an array of fusible links. Once programmed, the WORM devicecannot be reprogrammed.

Other forms of PROM devices include erasable PROM (“EPROM”) andelectrically erasable PROM (EEPROM) devices, which are alterable afteran initial programming. EPROM devices generally require an erase stepinvolving exposure to ultra violet light prior to programming thedevice. Thus, such devices are generally not well suited for use inportable electronic devices. EEPROM devices are generally easier toprogram, but suffer from other deficiencies. In particular, EEPROMdevices are relatively complex, are relatively difficult to manufacture,and are relatively large. Furthermore, a circuit including EEPROMdevices must withstand the high voltages necessary to program thedevice. Consequently, EEPROM cost per bit of memory capacity isextremely high compared with other means of data storage. Anotherdisadvantage of EEPROM devices is that although they can retain datawithout having the power source connected, they require relatively largeamounts of power to program. This power drain can be considerable in acompact portable system powered by a battery.

In view of the various problems associated with conventional datastorage devices described above, a relatively non-volatile, programmabledevice which is relatively simple and inexpensive to produce is desired.Furthermore, this memory technology should meet the requirements of thenew generation of portable computer devices by operating at a relativelylow voltage while providing high storage density, and a lowmanufacturing cost.

SUMMARY OF THE INVENTION

The present invention provides improved microelectronic devices for usein integrated circuits. More particularly, the invention providesrelatively non-volatile, programmable devices suitable for memory andother integrated circuits.

The ways in which the present invention addresses various drawbacks ofnow-known programmable devices are discussed in greater detail below.However, in general, the present invention provides a programmabledevice that is relatively easy and inexpensive to manufacture, and whichis relatively easy to program.

In accordance with one exemplary embodiment of the present invention, aprogrammable structure includes an ion conductor and at least twoelectrodes. The structure is configured such that when a bias is appliedacross two electrodes, one or more electrical properties of thestructure change. In accordance with one aspect of this embodiment, aresistance across the structure changes when a bias is applied acrossthe electrodes. In accordance with other aspects of this embodiment, acapacitance, or other electrical properties of the structure change uponapplication of a bias across the electrodes. One or more of theseelectrical changes may suitably be detected. Thus, stored informationmay be retrieved from a circuit including the structure.

In accordance with another exemplary embodiment of the invention, aprogrammable structure includes an ion conductor, at least twoelectrodes, and a barrier interposed between at least a portion of oneof the electrodes and the ion conductor. In accordance with one aspectof this embodiment the barrier material includes a material configuredto reduce diffusion of ions between the ion conductor and at least oneelectrode. The diffusion barrier may also serve to prevent undesiredelectrodeposit growth within a portion of the structure. In accordancewith another aspect, the barrier material includes an insulatingmaterial. Inclusion of an insulating material increases the voltagerequired to reduce the resistance of the device to its lowest possiblevalue. Devices including an insulating barrier may be well suited fornon-volatile memory (e.g., EEPROM) applications.

In accordance with another exemplary embodiment of the invention, aprogrammable microelectronic structure is formed on a surface of asubstrate by forming a first electrode on the substrate, depositing alayer of ion conductor material over the first electrode, and depositingconductive material onto the ion conductor material. In accordance withone aspect of this embodiment, a solid solution including the ionconductor and excess conductive material is formed by dissolving (e.g.,via thermal or photodissolution) a portion of the conductive material inthe ion conductor. In accordance with a further aspect, only a portionof the conductive material is dissolved, such that a portion of theconductive material remains on a surface of the ion conductor to form anelectrode on a surface of the ion conductor material.

In accordance with another embodiment of the present invention, at leasta portion of a programmable structure is formed within a through-hole orvia in an insulating material. In accordance with one aspect of thisembodiment, a first electrode feature is formed on a surface of asubstrate, insulating material is deposited onto a surface of theelectrode feature, a via is formed within the insulating material, and aportion of the programmable structure is formed within the via. Inaccordance with one aspect of this embodiment, after the via is formedwithin the insulating material, a portion of the structure within thevia is formed by depositing an ion conductive material onto theconductive material, depositing a second electrode material onto the ionconductive material, and, if desired, removing any excess electrode, ionconductor, and/or insulating material.

In accordance with a further exemplary embodiment of the invention,multiple bits of information are stored in a single programmablestructure.

In accordance with yet another exemplary embodiment of the presentinvention, a capacitance of a programmable structure is altered bycausing ions within an ion conductor of the structure to migrate.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims, considered inconnection with the figures, wherein like reference numbers refer tosimilar elements throughout the figures, and:

FIG. 1 is a cross-sectional illustration of a programmable structureformed on a surface of a substrate in accordance with the presentinvention;

FIG. 2 is a cross-sectional illustration of a programmable structure inaccordance with an alternative embodiment of the present invention;

FIG. 3 is a cross-sectional illustration of a programmable structure inaccordance with an alternative embodiment of the present invention;

FIG. 4 is a current-voltage diagram illustrating current and voltagecharacteristics of the device illustrated in FIG. 3 in an “on” and “off”state;

FIG. 5 is a cross-sectional illustration of a programmable structure inaccordance with yet another embodiment of the present invention;

FIG. 6 is a schematic illustration of a portion of a memory device inaccordance with an exemplary embodiment of the present invention; and

FIG. 7 is a schematic illustration of a portion of a memory device inaccordance with an alternative embodiment of the present invention.

DETAILED DESCRIPTION

The present invention generally relates to microelectronic devices. Moreparticularly, the invention relates to programmable structures suitablefor various integrated circuit applications.

FIG. 1 illustrates a programmable microelectronic structure 100 formedon a surface of a substrate 110 in accordance with an exemplaryembodiment of the present invention. Structure 100 suitably includeselectrodes 120 and 130 and an ion conductor 140.

Generally, structure 100 is configured such that when a bias greaterthan a threshold voltage (VT), discussed in more detail below, isapplied across electrodes 120 and 130, the electrical properties ofstructure 100 change. For example, in accordance with one embodiment ofthe invention, as a voltage V∃V_(T) is applied across electrodes 120 and130, conductive ions within ion conductor 140 begin to migrate and forman electrodeposit (e.g., electrodeposit 160) at or near the morenegative of electrodes 120 and 130. As the electrodeposit forms, theresistance between electrodes 120 and 130 decreases, and otherelectrical properties may also change. In the absence of any insulatingbarriers, which are discussed in more detail below, the thresholdvoltage required to grow the electrodeposit from one electrode towardthe other and thereby significantly reduce the resistance of the deviceis essentially the redox potential of the system, typically a fewhundred millivolts. If the same voltage is applied in reverse, theelectrodeposit will dissolve back into the ion conductor and the devicewill return to its high resistance state. As discussed in more detailbelow, structure 100 may be used to store information and thus may beused in memory circuits. For example, structure 100 or otherprogrammable structures in accordance with the present invention maysuitably be used in memory devices to replace DRAM, SRAM, PROM, EPROM,or EEPROM devices.

Substrate 110 may include any suitable material. For example, substrate110 may include semiconductive, conductive, semiinsulative, insulativematerial, or any combination of such materials. In accordance with oneembodiment of the invention, substrate 110 includes an insulatingmaterial 112 and a portion 114 including microelectronic devices formedon a semiconductor substrate. Layers 112 and 114 may be separated byadditional layers (not shown) such as, for example, layers typicallyused to form integrated circuits.

Electrodes 120 and 130 may be formed of any suitable conductivematerial. For example, electrodes 120 and 130 may be formed ofpolysilicon material or metal. In accordance with one exemplaryembodiment of the invention, electrodes 120 and 130 are formed of metal,and at least one of electrodes 120, 130 is formed of a metal such assilver, copper, or zinc that dissolves in ion conductor material 140.Having at least one electrode formed of a metal which dissolves in ionconductor 140 facilitates maintaining a desired dissolved metalconcentration within ion conductor 140, which in turn facilitates rapidand stable electrodeposit 160 formation within ion conductor 140 duringuse of structure 100.

In accordance with one embodiment of the invention, at least oneelectrode 120 and 130 is formed of material suitable for use as aninterconnect metal. For example, electrode 130 may form part of aninterconnect structure within a semiconductor integrated circuit. Inaccordance with one aspect of this embodiment, electrode 130 is formedof a material that is substantially insoluble in material comprising ionconductor 140. Exemplary materials suitable for both interconnect andelectrode 130 material include metals and compounds such as tungsten,nickel, molybdenum, platinum, metal silicides, and the like.

Alternatively, as illustrated in FIG. 2, a structure 200, includingelectrodes 220 and 230, and ion conductor 240, may include a barrierlayer (e.g., barrier 250), allowing one of electrodes 220, 230 to beformed of a material that dissolves in conductor 240. Barrier 250 maycomprise any material that restricts migration of ions between conductor240 and electrode 230. In accordance with exemplary embodiments of theinvention, barrier 250 includes titanium nitride, titanium tungsten, acombination thereof, or the like. In accordance with one aspect of thisembodiment, barrier 250 is electrically indifferent, i.e., it allowsconduction of electrons through structure 200, but it does not itselfcontribute ions to conduction through structure 200. An electricallyindifferent barrier may reduce undesired dendrite growth duringoperation of structure 200, and thus may facilitate an “erase” ordissolution of electrodeposit 160 when a bias is applied which isopposite to that used to grow the electrodeposit in the first instance.

Ion conductor 140 is formed of material that conducts ions uponapplication of a sufficient voltage. Suitable materials for ionconductor 140 include glasses and semiconductor materials. In oneexemplary embodiment of the invention, ion conductor 140 is formed ofchalcogenide material.

Ion conductor 140 may also suitably include dissolved conductivematerial. For example, ion conductor 140 may comprise a solid solutionthat includes dissolved metals and/or metal ions. In accordance with oneexemplary embodiment of the invention, conductor 140 includes metaland/or metal ions dissolved in chalcogenide glass. An exemplarychalcogenide glass with dissolved metal in accordance with the presentinvention includes a solid solution of As_(x)S_(1-x)—Ag,Ge_(x)Se_(1-x)—Ag, Ge_(x)S_(1-x)—Ag, As_(x)S_(1-x)—Cu,Ge_(x)Se_(1-x)—Cu, Ge_(x)S_(1-x)—Cu, other chalcogenide materialsincluding silver, copper, zinc, combinations of these materials, and thelike. In addition, conductor 140 may include network modifiers thataffect mobility of ions through conductor 140. For example, materialssuch as metals (e.g., silver), halogens, halides, or hydrogen may beadded to conductor 140 to enhance ion mobility and thus increaseerase/write speeds of the structure.

A solid solution suitable for use as ion conductor 140 may be formed ina variety of ways. For example, the solid solution may be formed bydepositing a layer of conductive material such as metal over an ionconductive material such as chalcogenide glass and exposing the metaland glass to thermal and/or photo dissolution processing. In accordancewith one exemplary embodiment of the invention, a solid solution ofAs₂S₃—Ag is formed by depositing As₂S₃ onto a substrate, depositing athin film of Ag onto the As₂S_(3,), and exposing the films to lighthaving energy greater than the optical gap of the As₂S₃,—e.g., lighthaving a wavelength of less than about 500 nanometers. If desired,network modifiers may be added to conductor 140 during deposition ofconductor 140 (e.g., the modifier is in the deposited material orpresent during conductor 140 material deposition) or after conductor 140material is deposited (e.g., by exposing conductor 140 to an atmosphereincluding the network modifier).

In accordance with one aspect of this embodiment, a solid solution ionconductor 140 is formed by depositing sufficient metal onto an ionconductor material such that a portion of the metal can be dissolvedwithin the ion conductor material and a portion of the metal remains ona surface of the ion conductor to form an electrode (e.g., electrode120). In accordance with alternative embodiments of the invention, solidsolutions containing dissolved metals may be directly deposited ontosubstrate 110.

An amount of conductive material such as metal dissolved in an ionconducting material such as chalcogenide may depend on several factorssuch as an amount of metal available for dissolution and an amount ofenergy applied during the dissolution process. However, when asufficient amount of metal and energy are available for dissolution inchalcogenide material using photodissolution, the dissolution process isthought to be self limiting, substantially halting when the metalcations have been reduced to their lowest oxidation state. In the caseof As₂S_(3,)—Ag, this occurs at Ag₄As₂S₃=2Ag₂S+As₂S, having a silverconcentration of about 44 atomic percent. If, on the other hand, themetal is dissolved in the chalcogenide material using thermaldissolution, a higher atomic percentage of metal in the solid solutionmay be obtained, provided a sufficient amount of metal is available fordissolution.

In accordance with one exemplary embodiment of the invention, at least aportion of structure 100 is formed within a via of an insulatingmaterial 150. Forming a portion of structure 100 within a via of aninsulating material 150 may be desirable because, among other reasons,such formation allows relatively small structures 100, e.g., on theorder of 10 nanometers, to be formed. In addition, insulating material150 facilitates isolating various structures 100 from other electricalcomponents.

Insulating material 150 suitably includes material that preventsundesired diffusion of electrons and/or ions from structure 100. Inaccordance with one embodiment of the invention, material 150 includessilicon nitride, silicon oxynitride, polymeric materials such aspolyimide or parylene, or any combination thereof.

A contact 160 may suitably be electrically coupled to one or moreelectrodes 120,130 to facilitate forming electrical contact to therespective electrode. Contact 160 may be formed of any conductivematerial and is preferably formed of a metal such as aluminum, aluminumalloys, tungsten, or copper.

A programmable structure in accordance with the present invention, e.g.,structure 100, may be formed in a variety of ways. In accordance withone embodiment of the invention, structure 100 is formed by formingelectrode 130 on substrate 110. Electrode 130 may be formed using anysuitable method such as, for example, depositing a layer of electrode130 material, patterning the electrode material, and etching thematerial to form electrode 130. Insulating layer 150 may be formed bydepositing insulating material onto electrode 130 and substrate 110, andforming vias in the insulating material using appropriate patterning andetching processes. Ion conductor 140 and electrode 120 may then beformed within insulating layer 150 by depositing ion conductor 140material and electrode 120 material within the via. Such ion conductorand electrode material deposition may be selective—i.e., the material issubstantially deposited only within the via, or the deposition processesmay be relatively non-selective. If one or more non-selective depositionmethods are used, any excess material remaining on a surface ofinsulating layer 150 may be removed, using, for example, chemicalmechanical polishing and/or etching techniques.

FIG. 3 illustrates a programmable structure 300 in accordance withanother embodiment of the present invention. Similar to structure 100,structure 300 is formed on a substrate 310 and includes electrodes 320and 330, and an ion conductor 340. In addition, structure 300 includesan insulating barrier 350 interposed between at least one electrode 320,330 and ion conductor 340. Insulating barrier 350 may be formed of anymaterial resistant to conduction of electricity. In accordance withvarious exemplary embodiments of the invention, barrier 350 is formed ofa metal oxide such as a native tungsten oxide or native nickel oxide.Alternatively other insulating materials may be deposited onto theelectrode. Among other things, barrier 350 may affect an effectivethreshold voltage of device 300 and prevent an electrical short betweenelectrode 320 and 330 via an electrodeposit (e.g., electrodeposit 360)unless a voltage is applied which is sufficiently high to cause theinsulating barrier to break down. For example, for a given insulatingmaterial, the effective threshold voltage of device 300 generallyincreases as a thickness of barrier 350 increase, thus device 300threshold voltage may be controlled, at least in part, by controllingbarrier 350 thickness. In this case, barrier 350 should be thin enough(i.e., 0 to about 3 nanometers) to allow electrons to tunnel throughbarrier 350 at a desired operating voltage (e.g., about 0.2V to about4V).

In operation, when a sufficient voltage is applied between two or moreelectrodes of a programmable structure (e.g., electrodes 320 and 330 ofstructure 300), electrodeposit 360 begins to form, through or along anedge of ion conductor 340, from the more negative electrode (cathode)(e.g., electrode 330) toward the more positive electrode (anode) (e.g.,electrode 320). For example, if electrode 330 is coupled to a negativeterminal of a voltage supply and electrode 320 is coupled to a positiveterminal of a voltage supply and a sufficient bias is applied betweenelectrodes 320 and 330, electrodeposit 360—e.g., a metallicdendrite—will begin to grow from electrode 330 toward electrode 320.

When electrode 330 is initially coupled to a more negative potential, anelectrodeposit begins to grow on surface 355 of barrier 350 uponapplication of a voltage ∃ the redox potential. As a voltage sufficientto breakdown barrier 350 is applied across electrodes 320 and 330 ashort forms between electrodes 320 and 330. When a sufficient reversebias is applied to electrodes 320 and 330, electrodeposit 360 dissolvesin conductor 340 and barrier 350 appears to heal itself such thatapproximately the same effective threshold voltage is required tobreakdown barrier 350. Thus, when structure 300 includes an insulatingbarrier 350, an effective threshold or “write” voltage is governed bybreakdown characteristics (e.g., thickness) of barrier-350.

Growth and configuration of an electrodeposit (e.g., electrodeposit 360)and reversal of electrodeposit growth generally affect electricalproperties of a programmable device such as structures 100-300. In turn,growth and a configuration of the electrodeposit depend on, among otherthings, an applied voltage bias, an amount of time the bias is appliedto electrodes (e.g., electrodes) 320 and 330, and structure geometry. Inparticular, at relatively low voltages, electrodeposit growth isrelatively slow and tends to concentrate about the cathode of astructure, whereas at higher voltages, the electrodeposit grows at afaster rate and tends to be more narrow and span a greater distancebetween the cathode and the anode, for a given amount of charge.

Once electrodeposit 360 begins to form, electrodeposit 360 willgenerally maintain its form after the voltage source is removed fromstructure 100. Thus, changes of electrical properties associated withgrowth of electrodeposit 360 (e.g., structure 300 capacitance,resistance, threshold voltage, and the like) do not vary substantiallyover time. In other words, the changes in electrical properties ofstructure 100 are relatively non-volatile. Accordingly, structure 100may be well suited for memory devices of electronic systems thattypically employ PROM, EPROM, EEPROM, FLASH devices, and the like.

In accordance with an alternate embodiment of the invention, theprogrammable structure may be periodically refreshed to enhance datastorage integrity. In this case, the structure may be employed in a RAM(e.g., DRAM) memory device.

Write Operation

Information may be stored using programmable structures of the presentinvention by manipulating one or more electrical properties of thestructures. For example, a resistance of a structure may be changed froma “0” or off state to a “1” or on state during a suitable writeoperation. Similarly, the device may be changed from a “1” state to a“0” state during an erase operation. In addition, as discussed in moredetail below, the structure may have multiple programmable states suchthat multiple bits of information are stored in a single structure.

FIG. 4 illustrates current-voltage characteristics of programmablestructure 300 in accordance with the present invention. For thestructure illustrated in FIG. 4, via diameter, D, is about 4 microns,conductor 340 is about 35 nanometers thick and formed of Ge₃Se₇—Ag (nearAs₈Ge₃Se₇), electrode 330 is indifferent and formed of nickel, electrode320 is formed of silver, and barrier 350 is a native nickel oxide. Asillustrated in FIG. 4, current through structure 300 in an off state(curve 410) begins to rise upon application of a bias of over about onevolt; however, once a write step has been performed (i.e., anelectrodeposit has formed), the resistance through conductor 340 dropssignificantly (i.e., to about 200 ohms), illustrated by curve 420 inFIG. 4. As noted above, when electrode 330 is coupled to a more negativeend of a voltage supply, compared to electrode 320, electrodeposit 360begins to form near electrode 330 and grow toward electrode 320. Aneffective threshold voltage (i.e., voltage required to cause growth ofelectrodeposit 360 and to break through barrier 350, thereby couplingelectrodes 320, 330 together) is relatively high because of barrier 350.In particular, a voltage V∃V_(T) must be applied to structure 300sufficient to cause electrons to tunnel through barrier 350 to form theeletrodeposit and to break down the barrier and conduct throughconductor 340 and at least a portion of barrier 350.

In accordance with alternate embodiments of the invention illustrated inFIGS. 1 and 2, an initial “write” threshold voltage is relatively lowbecause no insulative barrier is formed between, for example, ionconductor 140 and either of the electrodes 120, 130.

Read Operation

A state of the device (e.g., 1 or 0) may be read, without significantlydisturbing the state, by, for example, applying a forward or reversebias of magnitude less than a voltage threshold (about 1.4 V for astructure illustrated in FIG. 4) for electrodeposition or by using acurrent limit which is less than or equal to the minimum programmingcurrent (the current which will produce the highest of the on resistancevalues). A current limited (to about 1 milliamp) read operation is shownin FIG. 4. In this case, the voltage is swept from 0 to about 2 V andthe current rises up to the set limit (from 0 to 0.2 V), indicating alow resistance (ohmic/linear current-voltage) “on” state. Another way ofperforming a non-disturb read operation is to apply a pulse, with arelatively short duration, which may have a voltage higher than theelectrochemical deposition threshold voltage such that no appreciableFaradaic current flows, i.e., nearly all the current goes topolarizing/charging the device and not into the electrodepositionprocess.

Erase Operation

A programmable structure (e.g., structure 300) may suitably be erased byreversing a bias applied during a write operation, wherein a magnitudeof the applied bias is equal to or greater than the threshold voltagefor electrodeposition in the reverse direction. In accordance with anexemplary embodiment of the invention, a sufficient erase voltage(V∃V_(T)) is applied to structure 300 for a period of time which dependson the strength of the initial connection but is typically less thanabout 1 millisecond to return structure 300 to its “off” state having aresistance well in excess of a million ohms. Because structure 300 doesnot include a barrier between conductor 340 and electrode 320, athreshold voltage for erasing structure 300 is much lower than athreshold voltage for writing structure 300 because, unlike the writeoperation, the erase operation does not require electron tunnelingthrough barrier 350 or barrier 350 breakdown.

A portion of an integrated circuit 502, including a programmablestructure 500, configured to provide additional isolation fromelectronic components is illustrated in FIG. 5. In accordance with anexemplary embodiment of the present invention, structure 500 includeselectrodes 520 and 530, an ion conductor 540, a contact 560, and anamorphous silicon diode 570, such as a Schottky or p-n junction diode,formed between contact 560 and electrode 520. Rows and columns ofprogrammable structures 500 may be fabricated into a high densityconfiguration to provide extremely large storage densities suitable formemory circuits. In general, the maximum storage density of memorydevices is limited by the size and complexity of the column and rowdecoder circuitry. However, a programmable structure storage stack canbe suitably fabricated overlying an integrated circuit with the entiresemiconductor chip area dedicated to row/column decode, senseamplifiers, and data management circuitry (not shown) since structure500 need not use any substrate real estate. In this manner, storagedensities of many gigabits per square centimeter can be attained usingprogrammable structures of the present invention. Utilized in thismanner, the programmable structure is essentially an additive technologythat adds capability and functionality to existing semiconductorintegrated circuit technology.

FIG. 6 schematically illustrates a portion of a memory device includingstructure 500 having an isolating p-n junction 570 at an intersection ofa bit line 610 and a word line 620 of a memory circuit. FIG. 7illustrates an alternative isolation scheme employing a transistor 710interposed between an electrode and a contact of a programmablestructure located at an intersection of a bit line 710 and a word line720 of a memory device.

As noted above, in accordance with yet another embodiment of theinvention, multiple bits of data may be stored within a singleprogrammable structure by controlling an amount of electrodeposit whichis formed during a write process. An amount of electrodeposit that formsduring a write process depends on a number of coulombs or chargesupplied to the structure during the write process, and may becontrolled by using a current limit power source. In this case, aresistance of a programmable structure is governed by Equation 1, whereR_(on) is the “on” state resistance, V_(T) is the threshold voltage forelectrodeposition, and I_(LIM) is the maximum current allowed to flowduring the write operation. $\begin{matrix}{R_{on} = \begin{matrix}V_{T} \\{XXXX} \\I_{LIM}\end{matrix}} & {{Equation}\quad 1}\end{matrix}$

In practice, the limitation to the amount of information stored in eachcell will depend on how stable each of the resistance states is withtime. For example, if a structure is with a programmed resistance rangeof about 3.5 kΩ and a resistance drift over a specified time for eachstate is about ±250 Ω, about 7 equally sized bands of resistance (7states) could be formed, allowing 3 bits of data to be stored within asingle structure. In the limit, for near zero drift in resistance in aspecified time limit, information could be stored as a continuum ofstates, i.e., in analog form.

In accordance with yet another embodiment of the present invention, aprogrammable structure (e.g., structure 300) stores information bystoring a charge as opposed to growing an electrodeposit. In accordancewith one aspect of this embodiment, a capacitance of structure 300 isaltered by applying a bias to electrodes 320, 330 (e.g. positive voltageto electrode 320 with respect to electrode 330) such that positivelycharged ions migrate toward electrode 330. If the applied bias is lessthat a write threshold voltage (or voltage required to break throughbarrier 350), no short will form between electrodes 320 and 330.Capacitance of the structure 300 changes as a result of the ionmigration. When the applied bias is removed, the metal ions tend todiffuse away from barrier 350. However, an interface between conductor340 and barrier 350 is generally imperfect and includes defects capableof trapping ions. Thus, at least a portion of ions remain at orproximate an interface between barrier 350 and conductor 340. If a writevoltage is reversed, the ions may suitably be dispersed away from theinterface. A more complete description of a programmable structure inaccordance with this embodiment is provided in Application Ser. No.60/119,757, filed Feb. 11, 1999, the entire contents of which areincorporated herein by reference.

A programmable structure in accordance with the present invention may beused in many applications which would otherwise utilize traditionaltechnologies such as EEPROM, FLASH or DRAM. Advantages provided by thepresent invention over present memory techniques include, among otherthings, lower production cost and the ability to use flexiblefabrication techniques which are easily adaptable to a variety ofapplications. The programmable structures of the present invention areespecially advantageous in applications where cost is the primaryconcern, such as smart cards and electronic inventory tags. Also, anability to form the memory directly on a plastic card is a majoradvantage in these applications as this is generally not possible withother forms of semiconductor memories.

Further, in accordance with the programmable structures of the presentinvention, memory elements may be scaled to less than a few squaremicrons in size, the active portion of the device being less than onmicron. This provides a significant advantage over traditionalsemiconductor technologies in which each device and its associatedinterconnect can take up several tens of square microns.

Although the present invention is set forth herein in the context of theappended drawing figures, it should be appreciated that the invention isnot limited to the specific form shown. For example, while theprogrammable structure is conveniently described above in connectionwith programmable memory devices, the invention is not so limited. Forexample, the structure of the present invention may suitably be employedas a programmable active or passive devices within a microelectroniccircuit. Various other modifications, variations, and enhancements inthe design and arrangement of the method and apparatus set forth herein,may be made without departing from the spirit and scope of the presentinvention as set forth in the appended claims.

1. A microelectronic programmable structure comprising: a firstelectrode formed of a soluble material; an ion conductor comprising thesoluble material; a second electrode formed of an inert conductivematerial; a substrate disposed beneath the ion conductor; and aninsulating material formed overlying the substrate, wherein at least oneof the first electrode, the second electrode, and the ion conductor isat least partially formed within the insulating material.
 2. Themicroelectronic programmable structure of claim 1, wherein a thresholdvoltage of the structure is altered by applying a bias across the firstand second electrodes.
 3. The microelectronic programmable structure ofclaim 1, further comprising a barrier layer interposed between at leasta portion of one of the electrodes and the ion conductor.
 4. Themicroelectronic programmable structure of claim 3, wherein the barrierlayer is configured to reduce undesired electrodeposit growth within aportion of the structure.
 5. The microelectronic programmable structureof claim 1, wherein the first electrode is formed be depositing excessconductive material onto a surface of the ion conductor and exposing theexcess conductive material to a dissolution process to cause a portionof the excess conductive material to diffuse into the ion conductor. 6.The microelectronic programmable structure of claim 1, wherein the firstelectrode is formed on a surface of the substrate, the insolatingmaterial is formed overlying the substrate and the first electrode, andthe ion conductor is formed within an opening in the insulatingmaterial.
 7. The microelectronic programmable structure of claim 1,wherein the structure is configured to store multiple bits ofinformation.
 8. The microelectronic programmable structure of claim 1,wherein a capacitance of the structure is altered by applying a biasacross the first and the second electrode.
 9. A programmable deviceformed using the structure of claim 1, wherein the programmable devicesis selected from the group consisting of DRAM, SRAM, PROM, EPROM, andEEPROM.
 10. A microelectronic programmable structure comprising: a firstelectrode formed of a soluble material; an ion conductor comprising thesoluble material; a second electrode formed of an inert conductivematerial; a substrate disposed beneath the ion conductor; and amicroelectronic device formed using a portion of the substrate.
 11. Themicroelectronic structure of claim 10, wherein one of the first andsecond electrodes is formed of polysilicon material.
 12. Themicroelectronic structure of claim 10, wherein one of the first andsecond electrodes is formed of a material that dissolves in the ionconductor to alter an electrical property of the ion conductor.
 13. Themicroelectronic structure of claim 10, wherein one of the first andsecond electrodes is formed of interconnect material.
 14. Themicroelectronic structure of claim 13, wherein the interconnect materialcomprises a material selected from the group consisting of tungsten,nickel, platinum, and metal silicides.
 15. The microelectronic structureof claim 10, further comprising an electrically indifferent barrierlayer interposed between the ion conductor and at least apportion of oneof the first and the second electrodes.
 16. The microelectronicstructure of claim 10, wherein the first electrode is formed bydepositing excess conductive material onto a surface of the ionconductor and exposing the excess conductive material to a dissolutionprocess to cause a portion of the excess conductive material to diffuseinto the ion conductor, wherein the dissolution process is selected froma process comprising thermal dissolution and photo dissolution.
 17. Themicroelectronic structure of claim 10, further comprising a barrierlayer interposed between at least a portion of one of the first and thesecond electrode and the ion conductor, wherein the barrier layercomprises a material selected from the group consisting of siliconnitride, silicon oxynitride, polymeric material, or a combinationthereof.
 18. The microelectronic structure of claim 10, furthercomprising a contact electrically coupled to one of the first and thesecond electrode.
 19. The microelectronic structure of claim 18, whereinthe contact is formed of a material selected from the group consistingof aluminum, tungsten, and copper.
 20. The microelectronic structure ofclaim 10, further comprising a barrier layer interposed between at leasta portion of one of the first and the second electrode and the ionconductor, wherein the barrier layer is thin enough to allow electronsto tunnel through the barrier at a desired operating voltage.
 21. Themicroelectronic structure of claim 20, wherein the barrier layer has athickness less than about three nanometers.
 22. The microelectronicstructure of claim 10, further comprising: a p-n junction diode formedadjacent one of the first and the second electrode; and a contact formedadjacent the p-n junction diode.
 23. The microelectronic structure ofclaim 22, wherein the p-n junction diode is located at an intersectionof a word line and a bit line of a memory array.
 24. The microelectronicstructure of claim 10, wherein the microelectronic device comprisesmicroelectronic device comprises a circuit selected from groupconsisting of decode circuitry, sense amplifier, and data managementcircuitry.
 25. An analog memory device formed using the structure ofclaim
 10. 26. The microelectronic structure of claim 10, wherein thesubstrate comprises a plastic material.
 27. A smart card formed usingthe structure of claim
 26. 28. An inventory tag formed using thestructure of claim 26.